I believe that the conductor on board will offer you the sleeper at the lowest bucket fare for the sleeper! It doesn't matter what the fare that you paid was.

If you booked the sleeper with the ticket agent at the station just before you boarded (assuming it was available), you might pay the highest fare bucket for the room. But if you could get the same room from the conductor on board 5 minutes later, you would pay the lowest fare bucket for the room!